"Donald Trump might astound Americans on a routine basis, but we must draw a bright red line between political speech and suggestions of violence", she wrote in a statement jointly released with her husband Mark Kelly. "What is he trying to hide?"

Then-President Richard Nixon released his tax returns in 1973, starting an informal precedent. Ted Cruz, theorizing about why Trump has not made his tax returns public.

'If she gets to pick her judges, nothing you can do folks, ' Trump said Tuesday at a campaign rally in North Carolina.

Rasmussen's surveys have consistently shown Trump faring better than in other polls. Tim Kaine (D-Va.), will release new tax returns in the coming days, according to a person close to Clinton. Her tax rate has been above 40% every year since she left the state department in 2013.

Democrats have speculated that the returns could prove embarrassing to Trump in some way, perhaps by showing he is not as wealthy as he says. The documents showed that the Clintons reported earning $28.3 million in 2014 - more than double than in 2015 - and a total of more than $140 million between 2007 and 2014. Over the course of the last ten years, the couple donated donated 7.5 percent of their adjusted gross income to charity.

In 2015, the Clinton's paid $3.24 million in federal income taxes, amounting to effective tax rate as 34%, according to the Clinton campaign.